swc-project/swc · error

Unknown span attribute: {kind:?}

Error message

Unknown span attribute: {kind:?}

What it means

Compile-time panic from the Spanned derive support code (crates/ast_node/src/spanned.rs). Inside a `#[span(...)]` field attribute, the macro parses a comma-separated ident list (`InputFieldAttr`) and only `lo` and `hi` are valid kinds; any other ident panics with `Unknown span attribute: {kind:?}`.

Source

Thrown at crates/ast_node/src/spanned.rs:67

        for attr in &f.attrs {
            if !is_attr_name(attr, "span") {
                continue;
            }

            match &attr.meta {
                Meta::Path(..) => {}
                Meta::List(list) => {
                    let input = parse2::<InputFieldAttr>(list.tokens.clone())
                        .expect("failed to parse as `InputFieldAttr`");

                    for kind in input.kinds {
                        if kind == "lo" {
                            lo = true
                        } else if kind == "hi" {
                            hi = true
                        } else {
                            panic!("Unknown span attribute: {kind:?}")
                        }
                    }
                }
                _ => panic!("Unknown span attribute"),
            }
        }

        Self {
            ident: f.ident.clone(),
            ty: f.ty.clone(),
            lo,
            hi,
        }
    }
}

pub fn derive(input: DeriveInput) -> ItemImpl {
    let arms = Binder::new_from(&input)

View on GitHub (pinned to 5176682b65)

Solutions

1. Correct the attribute to exactly `#[span(lo)]` or `#[span(hi)]`
2. Mark one field `#[span(lo)]` and another `#[span(hi)]` to compose a span from both ends

Example fix

// before
struct Bin {
    #[span(low)]
    pub left: Expr,
}

// after
struct Bin {
    #[span(lo)]
    pub left: Expr,
}

When it happens

Trigger: Writing `#[span(low)]`, `#[span(lho)]`, or `#[span(lo, whole)]` on a field of a type deriving Spanned.

Common situations: Typos in span markers while porting AST node definitions; assuming extra span kinds (e.g. `both`, `full`) exist because `lo`/`hi` do.

Related errors

• Unknown span attribute
• #[derive(Spanned)] requires a field to get span from
• #[derive(Spanned)]: cannot determine span field to use for {
• #[derive(Spanned)]: #[span(lo)] and #[span(hi)] is required
• An element descriptor's .kind property must be either "metho

AI-assisted analysis of swc-project/swc@5176682b65 (2026-08-17). Data as JSON: /api/errors/5e0e25f3538b05c4. Report an issue: GitHub.